The present study is undertaken to define fine structural changes in cerebral cortex which may form the anatomical for the large group of cases with mental retardation in whom no gross developmental or destructive brain lesions are evident. Synaptic and dendritic development in cerebral cortex is studied, utilizing two histologic methods (the Golgi-Cox method for demonstration of dendritic aborization and the phosphotungstic acid method for synapses). Activity of Naion-Kion ATPase is measured as a biochemical correlate of dendritic and synaptic density. Factors that may control dendritic and synaptic growth in developing cerebral cortex, especially hormonal influenzas, are assessed in animal studies and in tissue culture of embryonic isocortex.